RE: Does control horn need to be in middle of control surface?
Flutter is harmonic vibration.
Think of a guitar string: pluck the string and it vibrates (Fig 1)
Place your finger lightly on the 12th fret and pluck the string. Both sides of the string will vibrate (Fig 2). Any guitar player out there can attest to the fact that this is called "Harmonics"
Now, place your finger lightly on the 10th or 11th fret and pluck the string. The string will give a dull "thunk" and stop vibrating (Fig 3).
The exact same thing happens with the control horn. Placeing it off-center will HELP to avoid flutter.